Gravity-driven tectonics at the dichotomy boundary (Mars)

The Martian crustal dichotomy is a prominent but enigmatic feature on Mars, expressed as a morpho-geologic boundary between the heavily cratered southern highlands and the relatively smooth northern lowlands, marking a strong topographic difference. The origins were ascribed to endogenic or exogenic mechanisms occurred at different stages of planet formation. The dichotomy would have subsequently being deformed by several processes such as Tharsis emplacement, extensive erosion, lateral crustal flow, fracturing and normal faulting [1, 2, 3].

The different morphology observed in the west, characterized by a smooth topographic transition (e.g., Arabia Terra), and the east (Nylosyrtis), characterized by a sharp prominent scarp, suggest a different evolution of the dichotomy (Fig. 1). In particular, the eastern dichotomy is dissected by extensional faults [1], distributed for 40–100 km in the highlands. In the corresponding lowlands, a set of compressional features (ridges) distributed parallel to the dichotomy, and, in our opinion, deviating from a pure concentric distribution around Utopia Planitia, are observed up to 1000 km north east of the eastern dichotomy. This is the only area on Mars where extensional faulting occur along and parallel to the dichotomy, associated with subparallel compressional structures in the lowlands; this suggest how this area may have undergone a different, more localized and complex tectonic history.

Fig. 1. Overview of the eastern dichotomy boundary showing the main extensional (black) and compressional (white) structures (from Tanaka et al., 2014), the topography and the study area (white box).

Such a configuration strongly resembles a terrestrial passive margin affected by gravity-driven deformation, in which the continental scarp is affected by an up-dip extensional domain (i.e., extension in the highlands and along the dichotomy) linked to a down-dip compressional domain, (sub)parallel to the extensional (i.e., ridges in the lowlands), through a relatively undeformed transitional domain. A possible gravitational relatively shallow origin for these normal faults have been already suggested but not investigated [2].

In this work we exploit the possibility that the northeastern dichotomy in the Nilosyrtis area was reshaped by gravity driven deformation, constrained by the age of the normal faults distributed along the dichotomy (Late Noachian/Early Hesperian) and of the compressional ridges observed in the lowlands (Early Hesperian). We perform high-resolution photogeological structural mapping (Fig. 2) integrated with 2D kinematic and numerical modelling aiming at characterize the timing of deformation, the depth and the nature of the common basal detachment and the possibility to have gravitational deformation under Martian gravity.

Fig. 2. Map (rotated 40° westwards) showing the gravity-driven fold-and-thrust belt. Its interpreted size is shown by the white polygon, including the detailed mapping of the up-dip extensional, the middle transitional and the down-dip compressional domains. The rose plots represents the orientations frequency of the extensional (light blue) and compressional structures (orange) in the north western, center and south eastern part of the system.

Kinematic modelling (Fig. 3) is applied to selected compressional structures aiming at characterize the depth to the basal detachment within the compressional domain and the amount of compression. To model the compressional structures characterizing the compressional domain, we propose a combined approach based on Area Depth Strain (ADS) [6] and Trishear modelling (TS) integrated with Fault Parallel Flow (FPF) [7]. Depth to detachment within the extensional domain is obtained from the intersection depth of the two faults delimiting grabens, assuming a fault dip of 60°. Indeed, simple grabens are delimited by two antithetic normal faults, whose intersection at depth represents a mechanical discontinuity.

Fig. 3. Resulting possible depths to detachment from the application of the TS, ADS, and grabens’ width methods. The sections are shown from northwest to southeast, being S1 (a), S2 (b), S3 (c), S4 (d) and S5 (e), respectively.

Numerical modelling (Fig. 4) is used to simulate the effect of viscosity, pore pressure and distribution of the common basal detachment in comparison with the development of the three structural domains observed in the study area, under Mars gravity. We provide three models: Model 1 (here not shown) represents a relatively frictional layer resembling an overpressured clay/shale detachment, Model 2 represents a weak viscous-like layer which would simulate a mixture of salt, ice and basalt debris. These two models assume a common and continuous basal detachment in both the highlands and the lowlands. Model 3 represents a more complex stratigraphy where a more frictional shallower detachment is simulated in the highlands while a weak viscous-like detachment is simulated in the lowlands.

Fig. 4. Strain change results of models 2 and 3, showing the main stages of deformation in which clear and observable changes in strain occur. The results are shown with a vertical exaggeration of 1.5.

The results of the applied methods indicate how the distribution of extensional and compressional structures observed in the Nilosyrtis area of the eastern dichotomy is supported by the presence of gravity-driven system. The system is deformed on top of a viscous basal detachment below the lowlands, linked to a more frictional basal detachment in the highlands.

The presence of a frictional detachment in the highlands would be linked to a possible sedimentary interlayer of i.e., eolian nature, between flood basalts. The presence of a weak detachment in the lowlands could be related to a more complex history, supporting the hypothesis of a giant Borealis impact, followed by the precipitation of evaporates from of a hypersaline sea, which may have been later reorganized by subsequent impacts and buried beneath volcanic deposits.

Based on high resolution remote sensing mapping, kinematic and mechanical modelling, we propose the existence of an ancient, partially buried and eroded gravity-driven fold-thrust belt, which occurred along the east dichotomy in the Nilosyrtis area, in a relatively short period comprised between Late Noachian–Early Hesperian. We propose to name it the Nilosyrtis fold-thrust belt.

[1] McGill and Dimitriou (1990). J. Geop. Res. 95, 12595–12605. [2] Nimmo (2005). Geology 33(7), 533–536. [3] Citron et al. (2018). Nature 555, 643–646. [4] Carboni et al. (2019). J. Str. Geol. 118, 210–223. [5] Carboni et al. (2025). Icarus 425, 116330.